Cyclic adsorptive gas purification processes typically employ one of two general classes of adsorption systems, namely: temperature swing adsorption (TSA) systems and pressure swing adsorption (PSA) systems. These adsorption systems typically contain two or more adsorbers that contain adsorbents for removal of impurities from a feed gas. The adsorbers are usually described to be operating in a production state also referred to as an adsorption state or in the regeneration state. The adsorber in the production state is also referred to as being on-line. The adsorber in the regeneration state is also referred to as being off-line. In the production state of both TSA and PSA systems, a feed gas stream is contacted with an adsorbent bed in the adsorber to produce a purified gas stream. The adsorber may contain one or more adsorbents. A given adsorbent selectively adsorbs one or more impurities present in the feed gas stream. At the end of the production state, the flow of feed gas to the adsorber is shut off. In the regeneration state of both TSA and PSA systems, the contaminant laden adsorbent bed is exposed to a flow of regeneration gas which facilitates desorption of impurities from the adsorbent and removal of desorbed impurities out of the adsorber. The regeneration gas in the regeneration state conventionally flows in a direction counter current to that of the feed gas flow in the production state. In the TSA system, the regeneration gas employed is a heated regeneration gas, provided at a temperature higher than that of the feed gas. Typically the temperature of heated regeneration gas is in the range of about 200° F. to about 600° F. The heated regeneration gas heats the adsorbent and facilitates regeneration of the adsorbent by desorption of impurities. The adsorbent has a lower adsorptive capacity at higher temperature. The heated regeneration gas also serves as a hot purge gas that removes the desorbed impurities from the adsorber. The PSA system in an air separation plant employs a waste nitrogen stream produced within the air separation plant as regeneration gas. The waste nitrogen is typically at a temperature close to the feed air temperature, and is provided to the PSA system at a pressure above the atmospheric pressure, sufficient to overcome the pressure drops and to be able to be discharged to the atmosphere. The adsorbed impurities in the PSA are desorbed due to the lower adsorptive capacity at lower pressures. The PSA regeneration gas serves as a purge gas that facilitates the regeneration of the adsorbent by desorption of impurities and removal of the desorbed impurities from the adsorber.
The cyclic adsorptive gas purification system can contain one or more adsorbers. By using at least two adsorbers in a parallel arrangement, the cyclic adsorptive gas purification system can be operated in a continuous mode; for example one adsorber can be operated in an adsorption state while the other adsorber is being regenerated and their roles are periodically reversed in the operating cycle, with equal periods being devoted to the adsorption state and to the regeneration state. Typically, such systems contain adsorbers that are substantially cylindrical in shape, and may have their axis with respect to feed flow as axial (vertical or horizontal), or of the radial type.
A conventional TSA process cycle for purifying air is generally described to contain the following steps: a) production of purified air by adsorption of impurities in feed air flowing through an adsorber at super atmospheric pressure and at ambient temperature for a pre-determined time period; b) initiating regeneration of the adsorbent by stopping the feed air flow and depressurizing the adsorber to a lower operating pressure, typically near atmospheric pressure; c) regeneration of the adsorbent in the depressurized adsorber by flowing a heated regeneration gas also referred to as hot purge gas for a pre-determined time period; an example of a heated regeneration gas is waste nitrogen produced in the air separation unit that is heated by means of one or more heaters/heat exchangers; d) cooling the regenerated adsorbent in the adsorber to push out residual heat in the adsorbent bed by flowing cool waste nitrogen; e) repressurizing the adsorber with purified air coming, for example, from another adsorber in the production phase; f) bringing the repressurized adsorber on-line and repeating steps (a) thru (e). Less conventionally, the regeneration may be carried out at a pressure substantially different from atmospheric pressure, either greater or even less than the ambient pressure by using suitable vacuum pumping means.
A conventional PSA process cycle for purifying air is usually described to contain: a) production of purified air by adsorption of impurities in feed air flowing through an adsorber at super atmospheric pressure for a pre-determined time period; b) initiating regeneration of the adsorbent by stopping feed air flow and depressurizing the adsorber to a lower operating pressure, typically near atmospheric pressure; c) regeneration of the adsorbent in the depressurized adsorber by flowing a purge gas for a pre-determined time period; an example of a purge gas is waste nitrogen produced in the air separation unit; d) repressurizing the adsorber with purified air coming, for example, from another adsorber in production phase; e) bringing the repressurized adsorber on-line and repeating steps (a) thru (d). The PSA process cycle is distinguished from the TSA process cycle in that the regeneration gas is not heated. Adsorbent bed cooling step is not required since the adsorbent doesn't get heated by the regeneration gas. The PSA cycle time is typically much shorter compared to the TSA cycle time.
Hybrid solutions such as thermally enhanced PSA (TEPSA) and thermal pressure swing adsorption (TPSA) have also been proposed as improvements to conventional PSA process cycle employed for air prepurification. A TEPSA system such as that described in U.S. Pat. No. 5,614,000 utilizes a two stage regeneration process in which previously adsorbed water is desorbed by PSA and at least a portion of previously adsorbed carbon dioxide is desorbed by TSA. In this process, desorption occurs by feeding a regeneration gas at a pressure lower than the feed stream and a temperature greater than the feed stream and subsequently replacing the hot regeneration gas by a cool regeneration gas. The heated regenerating gas allows the cycle time to be extended as compared to that of a conventional PSA system. However, the temperature of heated regeneration gas and cycle time of TEPSA is considerably lower than that of a conventional TSA.
The TPSA systems described in U.S. Pat. No. 5,885,650 and U.S. Pat. No. 5,846,295 relates to improvements to a conventional TSA. The adsorbent is first regenerated for a shorter time period using a regeneration gas heated to a much lower temperature and then purged with cool gas for a longer time period to desorb more of the remaining impurities loaded on the adsorbent.
The higher temperatures employed in TSA, TPSA and TEPSA systems may require the use of insulated vessels, a regeneration gas preheater and a precooler. The temperatures needed for the regenerating gas could range from about 200° F. to about 600° F., placing a more stringent and costly mechanical specification for the system, which increases costs. Typically, there will be more than one unwanted gas component which is removed in the process and generally one or more of these components will adsorb strongly than others on a particular adsorbent. The higher temperatures used for regenerating need to be sufficiently elevated for desorption of more strongly adsorbed component. In operation, there is extra energy cost associated with heating the regeneration gas.
Improvements to reduce energy costs have been proposed, for example U.S. Pat. No. 8,690,990 discloses a temperature swing adsorption process with two parallel purge flows, one through the sieve layer and the other through the alumina layer. The parallel purge step removes moisture from the moisture removal layer at the beginning of the regeneration cycle where as in the traditional TSA processes, the heated regeneration gas or hot purge gas first flows through the carbon dioxide removal layer followed by the water removal adsorbent layer; U.S. Pat. No. 5,766,311 describes yet another approach that uses multiple thermal pulses; U.S. Pat. No. 6,402,809 relates to an optimization approach that involves controlling, modifying and/or regulating the duration of cycle steps depending on at least one operating condition to minimize energy requirements; U.S. Pat. No. 7,846,237 describes yet another optimization approach that involves continuous monitoring of feed gas composition and accordingly adjusting cycle time to minimize energy usage.
The present invention aims to improve the known cyclic adsorptive gas purification processes, particularly TSA air purification process, by appreciably reducing the amount of energy consumed. The present invention modifies the regeneration state steps to save energy by using an unheated regeneration gas stream to desorb impurities prior to exposing the contaminant laden adsorbent bed to a heated regeneration gas stream.
Typically thermal swing adsorption (TSA) systems in cryogenic air separation plants also referred to as prepurification units or prepurifiers produce purified air for distillation at cryogenic temperatures by adsorbing impurities in feed air. These TSA systems utilize a heated desorption step to desorb the impurities and regenerate the adsorbent. The heated desorption step utilizes a hot purge gas that is at a temperature considerably higher than the feed gas temperature to promote desorption of impurities and regenerate the adsorbent. This is then followed by a cooling step that involves flowing a near ambient temperature gas to cool the adsorbent, push out the heat front through the adsorbent bed, and make it ready for adsorption step. The hot desorption step requires a significant amount of energy to desorb impurities and regenerate the adsorbent to a desired level. Any technology that would lower the energy requirements of the TSA process would have a significant economic benefit. Requiring less regeneration energy would allow for lower operating costs and the potential to design smaller heat exchangers and heaters which would save on capital costs. Thus the general problem to be solved is to reduce the energy usage in the regeneration state while achieving the same degree of impurities removal from feed air.
This invention aims at minimizing the regeneration state energy requirement by modifying the regeneration process steps, more particularly by introducing an unheated desorption step before the heated desorption step. The unheated desorption step can desorb certain amount of impurities, resulting in reduced amount of impurities to be desorbed by the heated desorption step, and hence reduced energy requirements. In the heated desorption step the adsorbent is heated to a pre-determined maximum temperature in the range of about 200° F. to about 600° F., preferably in the range of about 300° F. to about 500° F., and more preferably about 400° F. to assure regeneration of adsorbent to a desired level.
With these and other objects in mind, the invention is hereinafter described in detail and the novel features thereof being particularly pointed out in the appended claims.